Extrusion coating compositions and method

ABSTRACT

Solid extrusion coating compositions for metal substrates, a method of extrusion coating a metal substrate, and a metal article are disclosed. The extrusion coating composition is a thermoplastic material and comprises: (a) a first polyester having a weight average molecular weight of about 10,000 to about 80,000 and a Tg of greater than 45° C. to about 100° C., (b) a second polyester having a weight average molecular weight of about 10,000 to about 70,000 and a Tg of about −10° C. to about 45° C., and optionally, (c) a modifying resin, such as an epoxy resin having an epoxy equivalent weight of about 500 to about 15,000, wherein the Tg of the first and second polyesters differ by about 5 C.° to about 60 C.°. The extrusion coating composition is applied to a metal substrate in an extrusion process to provide a composition film having a thickness of about 1 to about 40 microns.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of PCT patentapplication Serial No. PCT/US97/08356, filed May 16, 1997, applicationSer. No. 09/117,837, filed Aug. 12, 1998 now U.S. Pat. No. 6,153,264,which is a continuation-in-part application of U.S. patent applicationSer. No. 08/649,480, filed May 17, 1996, abandoned.

FIELD OF THE INVENTION

The present invention relates to extrusion coating compositions formetal substrates that, after application, demonstrate excellentadhesion, weatherability, barrier properties, and flexibility; to amethod of extrusion coating a metal substrate; and to a metal article,such as a metal can or container, or a material of construction, such asaluminum siding, having at least one surface coated with an adherentlayer of an extrusion coating composition. An extrusion coatingcomposition comprises: (a) a first polyester having a weight averagemolecular weight of about 10,000 to about 80,000 and a glass transitiontemperature (Tg) of greater than 45° C. to about 100° C., (b) a secondpolyester having a weight average molecular weight of about 10,000 toabout 70,000 and a Tg of about −10° C. to about 45° C., and optionally,(c) a modifying resin, for example, an epoxy or phenoxy resin having anepoxy equivalent weight of about 500 to about 15,000, wherein the Tg ofthe first and second polyester differ by about 5 C.° to about 60 C.° Theextrusion coating composition is applied to a metal substrate as a filmhaving a thickness of about 1 to about 40 microns.

BACKGROUND OF THE INVENTION

It is well known that an aqueous solution in contact with an untreatedmetal substrate can result in corrosion of the untreated metalsubstrate. Therefore, a metal article, such as a metal container for awater-based product, like a food or beverage, is rendered corrosionresistant in order to retard or eliminate interactions between thewater-based product and the metal article. Generally, corrosionresistance is imparted to the metal article, or to a metal substrate ingeneral, by passivating the metal substrate, or by coating the metalsubstrate with a corrosion-inhibiting coating.

Investigators have sought improved coating compositions that reduce oreliminate corrosion of a metal article and that do not adversely affectan aqueous product packaged in the metal article. For example,investigators have sought to improve the imperviousness of the coatingin order to prevent corrosion-causing ions, oxygen molecules, and watermolecules from contacting and interacting with a metal substrate.Imperviousness can be improved by providing a thicker, more flexible,and more adhesive coating, but often, improving one advantageousproperty is achieved at the expense of a second advantageous property.

In addition, practical considerations limit the thickness, adhesiveproperties and flexibility of a coating applied to a metal substrate.For example, thick coatings are expensive, require a longer cure time,can be esthetically unpleasing, and can adversely affect the process ofstamping and molding the coated metal substrate into a useful metalarticle. Similarly, the coating should be sufficiently flexible suchthat the continuity of the coating is not destroyed during stamping andmolding of the metal substrate into the desired shape of the metalarticle.

Investigators also have sought coatings that possess chemical resistancein addition to corrosion inhibition. A useful coating for the interiorof a metal container is able to withstand the solvating properties of aproduct packaged in the metal container. If the coating does not possesssufficient chemical resistance, components of the coating can beextracted into the packaged product and adversely affect the product.Even small amounts of extracted coating components can adversely affectsensitive products, such as beer, by imparting an off-taste to theproduct.

Conventionally, organic solvent-based coating compositions were used toprovide cured coatings having excellent chemical resistance. Suchsolvent-based compositions include ingredients that are inherently waterinsoluble, and thereby effectively resist the solvating properties ofwater-based products packaged in the metal container. However, becauseof environmental and toxicological concerns, and in order to comply withincreasingly strict governmental regulations, an increasing number ofcoating compositions are water based. The water-based coatingcompositions include ingredients that are water soluble or waterdispersible, and, therefore, cured coatings resulting from water-basedcoating compositions often are more susceptible to the solvatingproperties of water.

In addition, water-based coating compositions do not completely overcomethe environmental and toxicological problems associated with organicsolvents because water-based compositions typically contain two or morepounds of organic solvent per gallon of coating composition. The organicsolvent is a necessary ingredient to dissolve and disperse compositioningredients, and to improve the flow and viscosity of the composition.Therefore, in order to entirely avoid the environmental andtoxicological problems associated with organic solvents, investigatorshave sought solid coating compositions that can be applied to a metalsubstrate. To date, investigators have had difficulty in providing asolid coating composition that matches a liquid coating composition withrespect to film uniformity, film appearance, and film performance.

In prior attempts to find a useful solid coating composition,investigators have tested powder coatings, laminated film coatings,radiation cure coatings, and extrusion coatings. A great deal ofresearch has been performed using free film laminates of polymers suchas polyethylene terephthalate (PET), polypropylene (PP), andpolyethylene (PE). In this method, a preformed polymer film, about 10 toabout 25 microns thick, is applied to the metal substrate. The filmlaminate method is a rapid method of coating a metal substrate, but themethod is expensive and the coated metal substrate does not possess allof the properties required, or desired, by can, can end, and closuremanufacturers.

Solid powder coatings also have been used to coat a metal substrate witha coating composition. However, the application of a thin, uniformcoating to a metal substrate, i.e., less than 40 microns, is difficultto impossible using the powder coating method. Often, if a thin coatingis applied to a metal substrate using a powder coating method, thecoating has imperfections which cause the film to fail. Such failuresare impermissible in the food and beverage container industry, whichfurther require thin coatings that can withstand shaping of a flat,coated metal substrate into a can, can end, or closure.

Solid coating compositions also have been extruded onto a metalsubstrate, for example, as disclosed in European Patent No. 0 067 060,PCT publication WO 94/01224, Smith et al. U.S. Pat. No. 5,407,702, andJones et al. U.S. Pat. No. 5,736,086. The extrusion coating of a solidcomposition onto a metal substrate is complicated by the fact that thesolid composition must be heated sufficiently to melt the compositionfor flow through the extrusion apparatus. The heating step either canalter the chemical make-up of the coating composition or can cause apremature cure of the coating composition, especially a thermosetcomposition, which changes the properties of the coating on the metalsubstrate or makes extrusion onto the metal substrate difficult due tocrosslinking in the extruder. Either effect can adversely affect theperformance of the composition coated on the metal substrate.

In order to overcome the problem of premature curing, investigators haveattempted to extrude thermoplastic coating compositions onto a metalsubstrate. These investigators also encountered serious problems, suchas composition components having either too high of a molecular weightfor easy, economical extrusion, or too low of a molecular weight therebyproviding an extruded film that is too soft for many practicalapplications, such as on the interior or exterior of a food or beveragecontainer. Therefore, many patents and publications in the field aredirected to extrusion apparatus and extrusion methods that permit theapplication of such solid coating compositions to a metal substrate.

Investigators, therefore, have sought a solid, extrudable coatingcomposition for use on the exterior and interior of food and beveragecontainers that exhibits the advantageous properties of adhesion,flexibility, chemical resistance, and corrosion inhibition, and that iseconomical and does not adversely affect the taste or other estheticproperties of sensitive foods and beverages packaged in the container.Investigators especially have sought useful extrusion coatingcompositions in order to reduce the environmental and toxicologicalconcerns associated with organic solvents. In particular, investigatorshave sought a solid, extrusion coating composition for food and beveragecontainers (1) that meets increasingly strict environmental regulations,(2) has corrosion inhibition properties at least equal to existingorganic solvent-based coating compositions, and (3) is easily extrudedonto a metal substrate as a thin, uniform film. Such an extrusioncoating composition would satisfy a long felt need in the art.

A present extrusion coating composition comprises: (a) a firstpolyester, (b) a second polyester, and optionally, (c) a modifyingresin, wherein the Tg of the first polyester differs from the Tg of thesecond polyester by about 5 C.° to about 60 C.° A present extrusioncoating composition is a thermoplastic composition and is extrudableonto a metal substrate. Therefore, a crosslinking step after extrusionof the composition onto the metal substrate, or use of a crosslinkingagent, is not required. A present extrusion coating composition is freeof organic solvents, yet an extruded film demonstrates excellent coatingproperties, such as adhesion, hardness, and flexibility.

A solid, extrusion coating composition of the present invention containsno organic solvents, and, therefore, overcomes the environmental andtoxicological problems associated with liquid coating compositions. Thepresent thermoplastic extrusion coating compositions also provide asufficiently flexible extruded coating such that the coated metalsubstrate can be deformed without destroying film continuity. Incontrast, thermosetting compositions often provide a rigid cured filmthereby making it difficult to impossible to coat the metal substrateprior to deforming, i.e., shaping, the metal substrate into a metalarticle, like a metal closure, can, or can end. Coating a metalsubstrate prior to shaping the metal substrate is the present standardindustrial practice.

As an added advantage, it is envisioned that a present extrusion coatingcomposition can be used on can ends, can bodies, and closures, therebyobviating the use of different coating compositions by containermanufacturers. Furthermore, a present extrusion coating compositionexhibits sufficient clarity, hardness, and mar resistance afterapplication for use as a coating on the exterior of a metal container.Accordingly, an extrusion coating composition of the present inventionhas a more universal range of applications, such as for the interiorcoating of a metal container for food or beverage products, or for theexterior coating of a metal container or a material of construction,such as aluminum siding; overcomes the environmental and toxicologicalconcerns associated with a liquid coating composition; and overcomesdisadvantages presented by other methods of applying a solid coatingcomposition to a metal substrate.

SUMMARY OF THE INVENTION

The present invention is directed to extrusion coating compositionsthat, after application to the metal substrate, effectively inhibitcorrosion of the metal substrate, do not adversely affect productspackaged in a container having an interior surface coated with thecomposition, and exhibit excellent flexibility, barrier properties,weathering, chemical resistance, and adhesion. An extrusion coatingcomposition of the present invention can be used on closures, can ends,and can bodies, and on container interiors and exteriors, as well asmaterials of construction, like aluminum siding and gutters. Anextrusion coating composition effectively inhibits corrosion of ferrousand nonferrous metal substrates when the composition is extruded to asurface of the metal substrate.

A present extrusion coating composition comprises: (a) a firstthermoplastic polyester, having a weight average molecular weight(M_(w)) of about 10,000 to about 80,000 and a Tg of greater than 45° C.to about 100° C., (b) a second thermoplastic polyester having an M_(w);of about 10,000 to about 70,000 and a Tg of about −10° C. to about 45°C., and optionally, (c) a modifying resin, such as an epoxy or phenoxyresin having an epoxy equivalent weight (EEW) of about 500 to about15,000, wherein the Tg of the first and second polyesters differ byabout 5 C.° to about 60 C.° The composition is free of organic solvents.

In particular, the present extrusion coating composition comprises: (a)about 10% to about 90%, by total weight of the composition, of a firstpolyester having an M_(w) of about 10,000 to about 80,000, andpreferably about 15,000 to about 60,000, (b) about 10% to about 90% bytotal weight of the composition, of the second polyester having an M_(w)of about 10,000 to about 70,000, and preferably about 15,000 to about50,000, and optionally, (c) 0% to about 25%, by total weight of thecomposition, of a modifying resin, for example, an epoxy or phenoxyresin having an EEW of about 500 to about 15,000, and preferably about1000 to about 10,000, or an acrylic resin having an M_(w) of about15,000 to about 100,000, or a polyolefin having an M_(w) of about 15,000to about 1,000,000, or a mixture thereof, wherein the Tg of the firstpolyester is about 5 C.° to about 60 C.°, and preferably about 15 C.° toabout 35 C.° greater than the Tg of the second polyester. To achieve thefull advantage of the present invention, the first and second polyestershave a Tg that differ by about 20 C.° to about 30 C.°. A presentextrusion coating composition optionally can include: (c) 0% to about50%, by total weight of the composition, of an inorganic filler, and (e)0% to about 4%, by total weight of the composition, of a flow controlagent.

The first and second polyesters included in a present extrusion coatingcomposition are thermoplastic polyesters prepared from an acid,preferably terephthalic acid, isophthalic acid, naphthane dicarboxylicacid, or a mixture thereof, and an aliphatic diol. At least onepolyester preferably is a poly(ethylene terephthalate) (PET) orco-polyester containing terephthalic acid and isophthalic acid. Otherpreferred polyesters are poly(butylene terephthalate) (PBT),poly(ethylene naphthalene-2,6-dicarboxylate) (PEN), poly(trimethyleneterephthalate) (PTT), and poly(trimethylene naphthanate) (PTN).

The polyesters have an acid value of 0 to about 150 mg (milligram) KOH(potassium hydroxide)/g (grams), a hydroxyl value of 0 to about 150 mgKOH/g, and a softening point of about 120° C. to about 200° C. Inaddition, the polyesters have a melt viscosity of about 200 to about3000 Pa.s (Pascal seconds), and a melt flow index (MFI) of about 800g/10 min (minutes) at 200° C. or about 5 g/10 min at 280° C. Inaccordance with an important feature of the present invention, the Tg ofthe first polyester is greater than the Tg of the second polyester byabout 5 C.° to about 60 C.°.

Components (a) and (b), and (c) and (d) and (e), if present, and otheroptional components are heated and intimately admixed to provide ahomogenous extrusion coating composition. After cooling, the extrusioncoating composition is comminuted into pellets having a particlediameter of about 1 to about 10 mm (millimeters), and preferably about 4to about 8 mm.

As used here and hereinafter, the term “extrusion coating composition”is defined as a solid coating composition including a first and secondpolyesters, an optional modifying resin, an optional filler, an optionalflow control agent, and any other optional ingredients. The term“extruded coating composition” is defined as an adherent polymericcoating resulting from extruding an extrusion coating composition onto ametal substrate.

Therefore, one important aspect of the present invention is to providean extrusion coating composition that effectively inhibits the corrosionof ferrous and nonferrous metal substrates. An extrusion coatingcomposition, after extrusion onto a metal substrate, provides anadherent barrier layer of an extruded coating composition thateffectively inhibits corrosion, exhibits excellent flexibility andadhesion on the metal substrate, and does not adversely affect aproduct, such as a food or beverage, that contacts the extruded coatingcomposition. Because of these advantageous properties, an extrudedcoating composition can be used to coat the interior of food andbeverage containers and overcome the disadvantages associated withconventional liquid compositions and with solid compositions applied bymethods such as powder coating and lamination. An extruded coatingcomposition comprises the first and second polyesters, and, if present,the modifying resin, the filler, and the flow control agent, essentiallyin the amounts these ingredients are present in the extrusion coatingcomposition.

In accordance with another important aspect of the present invention, anextruded coating composition demonstrates excellent flexibility andadhesion to a metal substrate. The excellent adhesion of an extrudedcoating composition to a metal substrate improves the barrier andcorrosion-inhibiting properties of the coating composition. Theexcellent flexibility of an extruded coating composition facilitatesprocessing of the coated metal substrate into a coated metal article,like in molding or stamping process steps, such that the cured coatingcomposition remains in continuous and intimate contact with the metalsubstrate. An extruded coating composition exhibits excellent chemicalresistance and does not adversely affect a food or beverage packaged ina container having an interior surface coated with the cured coatingcomposition. An extruded coating composition is sufficiently hard toresist scratching.

In accordance with another important aspect of the present invention, anextrusion coating composition of the present invention can be extrudedonto a metal substrate to provide a uniform film of extruded coatingcomposition having a film thickness of about 1 to about 40 microns, andpreferably 2 to about 30 microns. Uniform films of such a smallthickness have not been attainable using powder coating composition andmethods. In addition, a present extrusion coating composition can beused both on the interior and exterior of can bodies and can ends,thereby obviating the need for a container manufacturer to use multiplecoating compositions.

These and other aspects and advantages of the present invention willbecome apparent from the following detailed description of the preferredembodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An extrusion coating composition of the present invention, afterapplication to a metal substrate, provides an extruded coatingcomposition that effectively inhibits the corrosion of metal substrates,such as, but not limited to, aluminum, iron, steel and copper. Anextruded coating composition also demonstrates excellent adhesion to themetal substrate, excellent chemical resistance and scratch resistance,and excellent flexibility. An extruded coating composition does notimpart a taste to foods or beverages that contact the extruded coatingcomposition.

In general, a present extrusion coating composition comprises: (a) afirst polyester having an M_(w) of about 10,000 to about 80,000, and aTg of greater than 45° C. to about 100° C., and (b) a second polyesterhaving an M_(w) of about 10,000 to about 70,000, and a Tg of about −5°C. to about 45° C. The first polyester has Tg that is about 5 C.° toabout 60 C.° greater than the Tg of the second polyester. The extrusioncoating composition is a solid and is free of organic solvents.

An extrusion coating composition optionally can further include: (b) amodifying resin, such as an epoxy or phenoxy resin having an EEW ofabout 500 to about 15,000 and/or (c) a filler and/or (d) a flow controlagent. In addition, a present extrusion coating composition can includeoptional ingredients that improve the esthetics of the composition, thatfacilitate manufacture and/or extrusion of the composition, or thatimprove a functional property of the composition. The individualcomposition ingredients are described in more detail below.

(a) Polyesters

In accordance with an important feature of the present invention, anextrusion coating composition includes a first thermoplastic polyesterand a second thermoplastic polyester in a total amount of about 50% toabout 100%, by total weight of the composition. Preferably, an extrusioncomposition includes about 55% to about 90%, by total weight of thecomposition, of the first and second polyesters. To achieve the fulladvantage of the present invention, an extrusion coating compositionincludes about 60% to about 85% of the first and second polyesters, bytotal weight of the composition.

The first and second polyesters are present in the extrusion coatingcomposition in a weight ratio of first polyester to second polyester ofabout 9 to 1 to about 1 to 9, preferably from about 6 to 1 and about 1to 6. To achieve the full advantage of the present invention, the weightratio of first polyester to second polyester is about 3 to 1 to about 1to 3.

It should be understood that the first polyester can be a singlepolyester or a mixture of polyesters, as long as each polyestercomprising the first polyester has a Tg of about 45° C. to about 100° C.Similarly, the second polyester can be a single polyester or a mixtureof polyesters, as long as each polyester comprising the second polyesterhas a Tg of about −10° C. to about 40° C. Therefore, as used here andhereafter, the term “first polyester” or “second polyester” refers to asingle polyester or to a mixture of two or more polyesters.

Both the first and second polyesters are prepared from a dicarboxylicacid, preferably an aromatic dicarboxylic acid, and an aliphatic diol.These ingredients are interacted to provide a polyester having an M_(w)Kof about 10,000 to about 80,000, preferably of about 15,000 to about60,000, and to achieve the full advantage of the present invention,about 20,000 to about 50,000. Accordingly, the polyesters are consideredhigh molecular weight polyesters. The polyesters have an acid number ofabout 0 to about 150 mg KOH/g, and preferably about 5 to about 100 mgKOH/g. The polyesters have a hydroxyl number of 0 to about 150 mg KOH/g,and preferably about 5 to about 100 mg KOH/g.

Useful polyesters also possess properties that allow the polyesters tobe blended with the optional modifying resins and other compositioncomponents, to be extruded onto a metal substrate, and to provide anextruded coating composition having the necessary adhesion andflexibility to be applied to a metal substrate prior to shaping themetal substrate into a metal article. The polyesters also aresufficiently nonreactive such that, when the extrusion composition ismelted prior to and during extrusion, the polyesters do not enter acrosslinking reaction with the optional modifying resin or othercomposition components.

A polyester used in a present extrusion coating composition provides anextruded coating composition having good film tensile strength, goodpermeation resistance, retortability, and good barrier properties. Thepolyesters, and the extrusion coating composition, therefore, have asoftening point of 140° C. or greater, as measured using the procedureset forth in DIN 52011. Preferably, the polyesters and extrusion coatingcomposition have a softening point of 120° C. to about 200° C. Withinthis temperature range, the extruded coating compositions exhibitedimproved pasteurization/retortability resistance.

The first polyester has a Tg of greater than 45° C. to about 100° C.,and preferably about 50° C. to about 80° C. To achieve the fulladvantage of the present invention, the first polyester has a Tg ofabout 55° C. to about 75° C.

The second polyester has a Tg of about −10° C. to about 45° C.,preferably 0° C. to about 35° C. To achieve the full advantage of thepresent invention, the second polyester has a Tg of about 5° C. to about25° C.

The first and second polyesters have Tg's that differ by about 5 C.° to60 C.°, and preferably about 15 C.° to 35 C.°. To achieve the fulladvantage of the present invention, the difference in Tg's, or ΔTg,between the first and second polyesters is about 20° C. to about 30° C.In this ΔTg range, the blend of first and second polyesters issufficiently flexible to permit deformation of an extruded coatingcomposition without forming cracks, and is sufficiently hard to exhibitexcellent chemical and mar resistance. If the ΔTg of the first andsecond polyesters is less than about 5 C.°, the advantages of a blend ofpolyesters is not fully realized.

The Tg of the first and second polyester is measured by the followingprocedure. The ΔTg is simply the difference between the Tg of the firstpolyester and the Tg of the second polyester.

The glass transition temperature (Tg) of a polymer is the temperature atwhich an amorphous material changes from a brittle vitreous state to aplastic state. The Tg of a polyester was determined using a DifferentialScanning Calorimetry (DSC) instrument in a standard mode. In particular,the method utilized a TA Instruments Model 2920 DSC instrument, with ahelium flow gas of 35 cm³/minute. Data was collected on a TA Instruments3100 Thermal Analyst Computer using an indium standard for temperaturecalibration.

An indium standard first was prepared in accordance with ISO9000calibration documentation. A small piece of indium was cut from thestock standard, and placed into an aluminum DSC sample pan with cover,then crimped closed. The standard was heated from 100° C. to 180° C. at20 C.° per minute. A helium gas purge at 35 cc per minute was used. Themelting point and heat of melt calculations were made of the indiumstandard, and the results were entered into the calibration file storedin the Thermal Analyst program on the TA 3100 computer.

A sealed DSC pan containing a polyester sample was placed into the DSCinstrument at room temperature. The DSC heating chamber cover wasclosed. The sample was cooled to −40° C. using either liquid nitrogen inthe cooling can or the RSC cooling system. After equilibration of theDSC was reached, the sample was heated at 20° C. per minute through theTg of the sample by about 10° C. and then cooled to −40° C. again. Thesample was reheated at 20 C.° per minute through the glass transitiontemperature and the Tg was determined. The glass transition temperaturewas calculated at the temperature at the mid-point in the change in heatcapacity.

Useful first and second polyesters also exhibit a melt viscosity ofabout 200 to about 3000 Pa.s (Pascal seconds). The melt viscosity ismeasured using a cone/plate viscometer by the standard PIN I50 1133procedure. The melt flow index (MFI), as measured using DIN 53735, of auseful polyester is about 800 g/10 min. at 200° C. or about 5 g/10 min.at 280° C.

The first and second polyesters typically are prepared by condensing adicarboxylic acid with an aliphatic diol. To provide polyesters havingoptimum properties for an extrusion coating composition for a food orbeverage container, the dicarboxylic acid preferably is an aromaticdicarboxylic acid. To achieve the full advantage of the presentinvention, the dicarboxylic acid comprises terephthalic acid,isophthalic acid, a naphthalene dicarboxylic acid, and mixtures thereof.It is also understood that an esterifiable derivative of a dicarboxylicacid, such as a dimethyl ester or anhydride of a dicarboxylic acid, canbe used to prepare the polyester.

In particular, exemplary dicarboxylic acids used to prepare the firstand second polyesters include aliphatic and aromatic dicarboxylic acids,such as, but not limited to, phthalic acid, isophthalic acid,terephthalic acid, adipic acid, malonic acid,2,6-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid,hexahydroterephthalic acid, 1,4-cyclohexanedicarboxylic acid, sebacicacid, azeleic acid, succinic acid, glutaric acid, fumaric acid, adipicacid, and mixtures and esterifiable derivatives thereof. Substitutedaliphatic and aromatic dicarboxylic acids, such as halogen oralkyl-substituted dicarboxylic acids, also are useful. Preferably, atleast 60 mol % aromatic dicarboxylic acids are used to prepare thepolyester.

Exemplary, but nonlimiting, diols used to prepare the first and secondpolyesters include ethylene glycol, diethylene glycol, triethyleneglycol, propylene glycol, dipropylene glycol, hexylene glycol, butyleneglycol, pentylene glycol, neopentyl glycol, trimethylpropane diol,1,4-cyclohexanedimethanol, 1,10-decanediol,2,2-dimethyl-1,3-propanediol, 2,2,4,4-tetramethyl-1,3-cyclobutendiol, apolyethylene or polypropylene glycol having a molecular weight of about500 or less, and mixtures thereof. A small amount of a triol or polyol,i.e., 0 to 3 mole % of diol, can be used to provide a partiallybranched, as opposed to linear, polyester.

The diol and the dicarboxylic acid, in correct proportions, areinteracted under standard esterification procedures to provide a firstand second polyester having the necessary M_(w), Tg, molecular weightdistribution, branching, crystallinity, and functionality for use in apresent extrusion coating composition. Examples of useful polyesters canbe prepared as set forth in Brünig et al. U.S. Pat. No. 4,012,363,incorporated here by reference, and in Canadian Patent No. 2,091,875.

In addition, useful polyesters are commercially available under thetradename DYNAPOL, from Hüls AG, Berlin, Germany. Examples of specificpolyesters are DYNAPOL P1500, DYNAPOL P1510, and DYNAPOL P1550, eachavailable from Hüls AG and based on terephthalic acid and/or isophthalicacid. Another useful class of polyesters is the GRILESTA polyesters,like GRILESTA V 79/20, available from EMS. Other useful commercialpolyesters include, but are not limited to, SHELL CARIPAK P76, availablefrom Shell Chemicals (Europe), Switzerland; SELAR polyesters, like SELARPT 6129 and SELAR PT 8307, both available from DuPont Packaging andIndustrial Polymers, Wilmington, Del. The CRASTIN polyesters, likeCRASTIN 6129, available from DuPont, also can be used. CELANEX 3100 andHOECHST 1700A also are useful polyesters.

A polyester also can be prepared by condensing a dicarboxylic acid orderivative of a dicarboxylic acid described above with a low molecularweight epoxy compound. The low molecular weight epoxy compound containsan average of abut 1.5 to about 2.5 epoxy groups per molecule and has anEEW of abut 150 to about 500. An exemplary low molecular weight epoxycompound is EPON 828, available from Shell Chemical Co., Houston, Tex.

Especially useful polyesters include polyethylene terephthalates (PET),polybutylene terephthalates (PET), polyethylene naphthanates (PEN), andpolybutylene naphthalates (PBN), polytrimethylene terephthalate (PTT),polytrimethylene naphthanate (PTN), and mixtures thereof.

(b) Optional Modifying Resin

The extrusion coating composition also includes 0% to about 25%, bytotal weight of the composition, of an optional modifying resin.Preferably, an extrusion composition contains about 2% to about 20% ofan optional modifying resin, by total weight of the composition. Toachieve the full advantage of the present invention, the extrusioncomposition contains about 8% to about 15% of an optional modifyingresin, by total weight of the composition.

The optional modifying resin does not substantially react with thepolyester during manufacture of the extrusion coating composition orduring the extrusion process. Accordingly, after application to a metalsubstrate, the extrusion coating composition is not subjected a curingstep. The modifying resin, however, improves the barrier properties ofthe extruded coating and the adhesion of the extruded coatingcomposition to the metal substrate.

One useful modifying resin is an epoxy or phenoxy resin having an EEW ofabout 500 to about 15,000, and preferably about 1000 to about 10,000. Toachieve the full advantage of the present invention, the epoxy orphenoxy resin has an EEW of about 2000 to about 8000. Within the aboveEEW range for the epoxy or phenoxy resin, the extruded coatingcomposition is sufficiently flexible to permit deformation of anextruded coating composition without forming cracks, and is sufficientlyhard to exhibit excellent chemical and mar resistance.

Preferably, the epoxy or phenoxy resin is a solid material that can bemelted and admixed with a molten polyester to provide an extrusioncoating composition of the present invention. Preferred epoxy andphenoxy resins contain an average of about 1.5 to about 2.5 epoxy groupsper molecule of epoxy resin, but epoxy novolac resins containing greaterthan about 2.5 epoxy groups per molecule also can be used, i.e.,containing about 2.5 epoxy groups to about 6 epoxy groups.

The epoxy or phenoxy resin can be an aliphatic resin or an aromaticresin. The preferred epoxy and phenoxy resins are aromatic, like epoxyand phenoxy resins based on the diglycidyl ether of bisphenol A orbisphenol F. An epoxy resin can be used in its commercially availableform, or can be prepared by advancing a low molecular weight epoxycompound by standard methods well known to those skilled in the art.

Exemplary epoxy resins include, but are not limited to, EPON 1004, EPON1007, and EPON 1009, all available from Shell Chemical Co., Houston,Tex., or ARALDITE® 6099, available from CIBA-GEIGY Corp., Ardsley, N.Y.

In general, suitable epoxy and phenoxy resins are aliphatic-,cycoaliphatic-, or aromatic-based epoxy resins, such as, for example,epoxy resins represented by structural formulae I and II:

wherein each A is, independently, a divalent hydrocarbyl group having 1to about 12, preferably 1 to about 6, and most preferably 1 to about 4,carbon atoms; each R is, independently, hydrogen or an alkyl grouphaving 1 to about 3 carbon atoms; each X is, independently, hydrogen, ahydrocarbyl or hydrocarbyloxy group having 1 to about 12, preferably 1to about 6, and most preferably 1 to about 4, carbon atoms, or a halogenatom, preferably chlorine or bromine; n is 0 or 1, and n′ has an averagevalue of about 2 to about 30, and preferably 10 to about 30.

In particular, the preferred epoxy and phenoxy resins are the(diglycidyl ether/bisphenol-A) resins, i.e., polyether diepoxidesprepared by the polymeric adduction of bisphenol-A (III)

and the diglycidyl ether of bisphenol-A (IV).

In this case, the epoxy resin is a mixture including polymeric speciescorresponding to different values of n′ in the following idealizedformula V:

wherein n′ is a number from about 2 to about 30.

In addition to bisphenol-A, useful epoxy and phenoxy resins can beprepared by advancing a diglycidyl ether of a bisphenol listed belowwith an exemplary, but nonlimiting, bisphenol listed below:

Presently, governmental agencies are issuing regulations directed to theamount of free epoxy groups in coatings present on food and beveragecontainers and closures. Therefore, for some applications, an epoxyresin is not a suitable modifying resin. In these applications, anacrylic resin or a polyolefin resin can be used as the optionalmodifying resin. A mixture of an epoxy resin, an acrylic resin, and apolyolefin resin also can be used.

The acrylic resin has an M_(w) of about 15,000 to about 100,000, andpreferably about 20,000 to about 80,000. Acrylic resins include, but arenot limited to, homopolymer and copolymers of acrylic acid, methacrylicacid, esters of acrylic acid, esters of methacrylic acid, acrylamides,and methacrylamides.

The polyolefin resin has an M_(w) of about 15,000 to about 1,000,000,and preferably about 25,000 to about 750,000. Polyolefin resins include,but are not limited to, homopolymers and copolymers of ethylene,propylene, ethylene-propylene blends, 1-butene, and 1-pentene. Thepolyolefin also can contain functionalized olefins, such as an olefinfunctionalized with hydroxy or carboxy groups.

(c) Optional Inorganic Filler

To achieve the full advantage of the present invention, an extrusioncoating composition includes 0% to about 50%, preferably 0% to about30%, and most preferably 0% to about 25%, by total weight of thecomposition, of an inorganic filler. An inorganic filler is included toimprove the physical properties of an extruded coating composition.

Exemplary inorganic fillers used in the coating composition of thepresent invention include, but are not limited to, clay, mica, aluminumsilicate, fumed silica, magnesium oxide, zinc oxide, barium oxide,calcium sulfate, calcium oxide, aluminum oxide, magnesium aluminumoxide, zinc aluminum oxide, magnesium titanium oxide, iron titaniumoxide, calcium titanium oxide, and mixtures thereof. The inorganicfiller is essentially nonreactive and is incorporated into the extrusioncoating composition in the form of a powder, generally about 10 to 200microns in diameter, and in particular, about 50 microns to about 125microns in diameter.

(d) Optional Flow Control Agent

An extrusion coating composition of the present invention also cancontain a flow control agent to assist in achieving a uniform film ofextruded coating composition on the metal substrate. The flow controlagent is present in an amount of 0% to about 6%, and preferably 0% toabout 5%, by total weight of the composition.

An exemplary, but nonlimiting, flow control agent is a polyacrylateavailable from Henkel Corporation, as PERENOL F 30 P. Another usefulpolyacrylate flow control agent is ACRYLON MFP. Numerous other compoundsand other acrylic resins known to persons skilled in the art also can beused as a flow control agent.

(e) Other Optional Ingredients

An extrusion coating composition of the present invention also caninclude other optional ingredients that do not adversely affect theextrusion coating composition or an extruded coating compositionresulting therefrom. Such optional ingredients are known in the art, andare included in an extrusion coating composition to enhance compositionesthetics, to facilitate manufacturing and application of the extrusioncoating composition, and to further improve a particular functionalproperty of an extrusion coating composition or an extruded coatingcomposition resulting therefrom.

Such optional ingredients include, for example, dyes, pigments,anticorrosion agents, antioxidants, adhesion promoters, lightstabilizers, and mixtures thereof. Each optional ingredient is includedin a sufficient amount to serve its intended purpose, but not in such anamount to adversely affect an extrusion coating composition or anextruded coating composition resulting therefrom.

For example, a pigment, in an amount of 0% to about 50% by weight of thecomposition, is a common optional ingredient. A typical pigment istitanium dioxide, barium sulfate, carbon black, or an iron oxide. Inaddition, an organic dye or pigment can be incorporated in the extrusioncoating composition.

In addition, an additional polymer, i.e., a second modifying polymer,can be added to the extrusion coating composition to improve theproperties of the extruded coating composition. The second modifyingpolymer preferably is compatible with the other composition componentsand does not adversely affect the extruded coating composition. Toachieve a coated metal substrate having a nongloss finish, the secondmodifying polymer can be substantially incompatible with the polyesterand optional modifying polymer. The second modifying polymer can be athermoplastic or a thermoset polymer, and is present in the extrusioncoating composition in an amount of 0% to about 50%, and preferably 0%to about 20%, by total weight of the composition.

Nonlimiting examples of optional second modifying polymers that can beincorporated into the extrusion coating composition are a carboxylatedpolyester, a carboxylated polyolefin, a polyamide, a fluorocarbon resin,a polycarbonate, a styrene resin, an ABS(acrylonitrile-butadiene-styrene) resin, a chlorinated polyether, aurethane resin, and similar resins. Polyamide resins include nylon 66,nylon 6, nylon 610, and nylon 11, for example. A useful polyolefin ispolyethylene or polypropylene, including homopolymers and copolymers,for example. Fluorocarbon resins include tetrafluorinated polyethylene,trifluorinated monochorinated polyethylene, hexafluorinatedethylene-propylene resin, polyvinyl fluoride, and polyvinylidenefluoride, for example. However, even if an optional second modifyingpolymer is added to the extrusion coating, the extrusion coatingcomposition is free of a crosslinking agent and is not subjected to acuring step after extrusion onto a metal substrate.

An extrusion coating composition of the present invention can beprepared by methods well known in the art, such as by individuallyheating the first polyester, the second polyester, and the optionalmodifying resin to a sufficient temperature to melt each ingredient,then admixing the molten polyesters and optional modifying resin, suchas in a single screw or double screw extruder, to provide a uniformextrusion coating composition. Optional ingredients can be added to theextrusion coating composition either by incorporation into one of themolten ingredients prior to admixture of the molten ingredients, or canbe added to the molten extrusion coating composition after ingredientshave been admixed. If an optional second modifying polymer is present inthe composition, the second modifying polymer is melted and added to themolten extrusion coating composition at any convenient step of themanufacturing process. Alternatively, all composition ingredients can beadmixed in the solid state, followed by melting the resulting admixtureand extrusion, to provide a uniform molten composition.

After a uniform molten composition is prepared, the extrusion coatingcomposition is allowed to cool and solidify. The resulting extrusioncoating composition then is formed into pellets having a particlediameter of about 1 to about 10 mm. The pellets are stored and kept dryuntil use in an extrusion process. Preferably, the pellets are subjectedto a heating step prior to extrusion in order to expel any waterabsorbed by the extrusion coating composition during storage.

To demonstrate the usefulness of an extrusion coating composition of thepresent invention, the following examples were prepared, then extrudedonto a metal substrate to provide a coated metal substrate. The coatedmetal substrates then were tested for use as a food or beveragecontainer. The extruded coatings were tested for an ability to inhibitcorrosion of a metal substrate, for adhesion to the metal substrate, forchemical resistance, for flexibility, and for scratch and marresistance. Examples 1 through 31 illustrate some important features andembodiments of an extrusion coating composition of the presentinvention, and illustrate methods of extruding a coating composition ofthe present invention. Examples A and B are comparative examplescontaining a single polyester resin.

Comparative Examples A and B Ingredient A (wt. %) B (wt. %) DYNAPOL P1500¹⁾ 90.55 99.45 ARALDITE ® GT 6099²⁾ 8.90 0 ACRYLON MFP³⁾ 0.55 0.55¹⁾A polyester available from Hüls AG, having softening point of 170-176°C., a Tg of about 23° C., and a melt viscosity of about 70-80 Pa.s at240° C.; ²⁾An epoxy resin available from CIBA-GEIGY having an EEW ofabout 2500 to about 4000; and ³⁾An acrylic flow central agent.

The extrusion coating compositions of Comparative Examples A and B wereprepared by melting the polyester and adding the flow control agent,with stirring, to the molten polyester. The resulting mixture was heatedto maintain the polyester in the molten state. Then, the previouslymelted epoxy resin was admixed with the molten polyester by passing theepoxy resin and polyester through a twin blade extruder. The resultingcompositions of Comparative Examples A and B were allowed to cool toroom temperature and solidify. The solid compositions then were formedinto pellets, the majority having a particle diameter of about 1 toabout 10 mm. The extrusion coating compositions of Comparative ExamplesA and B had a melt flow index (MFI) of 62.2 g/10 minutes at 200° C., anda melting peak of 172.2° C. (determined by differential scanningcalorimetry (DSC)).

The compositions of Examples 1-31 were prepared in an essentiallyidentical manner to the comparative compositions of Examples A and B,except a second polyester resin is included in a majority of Examples1-31. Examples 2, 8, and 16 contain a single polyester, and arecomparative examples. Examples 30 and 31 contain a single polyester, andare comparative examples. Examples 30 and 31 contain additionalcomposition ingredients, for example, a pigment, and are prepared in anessentially identical manner as Comparative Examples A and B.

Ex. Ex. 2 Ex. Ex. 8 Ex. Ex. Ex. Ex. Ex. Ingredient 1⁴⁾ (comparative) Ex.3 Ex. 4 Ex. 5 Ex. 6 7⁴⁾ (comparative) Ex. 9 10 11 12 13⁴⁾ 14 15 CRASTIN47.25 100.00 89.75 74.75 49.75 24.75 47.25 100.00 89.75 74.75 49.7524.75 6129⁵⁾ DYNAPOL 47.25  9.75 24.75 49.75 74.75 47.25  9.75 24.7549.75 74.75 23.62 47.25 40.87 1500¹⁾ ARALDITE ®  5.00  5.00  5.01  5.00 5.01 GT6099²⁾ ACRYLON MFP³⁾  0.50  0.50  0.50  0.50  0.50  0.50  0.50 0.50  0.50  0.50  0.50  0.50  0.50 CARIPAK P76⁶⁾ 47.25 100.00 89.7574.75 49.75 24.75 SELAR PT8307⁷⁾ 70.87 47.25 23.62 Ex. 16 Ex. Ex. Ex.Ex. Ex. Ingredient (Comparative) Ex. 17 Ex. 18 Ex. 19 Ex. 20 Ex. 21⁴⁾Ex. 22 Ex. 23 Ex. 24 25 26 27 28 29 CRASTIN 74.75 49.75 24.75 74.7549.75 24.75 64.75 39.75 14.75 6129⁵⁾ DYNAPOL  9.75 24.75 49.75 74.7520.00 20.00 20.00 1500¹⁾ ARALDITE ® GT6099 ² ACRYLON MFP³⁾  0.50  0.50 0.50  0.50  0.50  0.50  0.50  0.50  0.50  0.50  0.50  0.50  0.50CARIPAK P76⁶⁾ 24.75 49.75 74.75 14.75 39.75 14.75 SELAR PT8307⁷⁾ 100.0089.75 74.75 49.75 24.75 24.75 49.75 74.75 ⁴⁾Percent by weight in thecomposition; and ⁵⁾A polybutylene terephthalate polyester, availablefrom DuPont Packaging and Industrial Polymers, Wilmington, DE, having amelting point of 225° C., a Tg of about 50° C., and a melt flow index ofabout 6 to 8 g/10 min. at 240° C. ⁶⁾A polyethylene terephthalatepolyester, available from Shell Chemicals (Europe), Switzerland, havinga melting point of 250° C. and a Tg of about 70° C. ⁷⁾A modifiedpolyester available from DuPont Packaging and Industrial Polymers,Wilmington, DE, having a melting point of 220° C., a Tg of about 70° C.,and a melt viscosity of about 560 Pa.s at 280° C.

The following Examples 30 and 31 are pigmented embodiments of theextrusion coating compositions.

Ingredient Ex. 30⁴⁾ Ex. 31 SELAR PT8307 20.00 36.25 Titanium Dioxide⁸⁾12.00 20.00 DYNAPOL 1500¹⁾ 58.55 CRASTIN 6129⁵⁾ 36.25 Wax⁹⁾ 2.00 DYNAPOL1510¹⁰⁾ 5.00 ARALDITE GT6099²⁾ 8.90 ACRYLON MFP³⁾ 0.55 0.50 ⁸⁾Pigment;⁹⁾Hoechst wax C; and ¹⁰⁾A polyester, available from Hüls AG, having amelting point of 147-154° C., a Tg of about 23° C., a melt viscosity of35-40 Pa.s at 240° C., and a melt index of about 120 g/10 min at 200° C.

The first test performed on extrusion coating compositions of thepresent invention was to determine whether the first and secondpolyesters retained their individual identities (i.e., their distinctTg's) during the processes of manufacturing the composition andextruding the composition onto a metal substrate, or whether atransesterification reaction occurred to form an alloy having a singleTg. In this test, eight separate samples of extrusion coatingcompositions were treated in an extruder at 300° C. for various lengthsof time. The samples contained either a blend of DYNAPOL 1500 andCARIPAK P76, or a blend of DYNAPOL 1500 and a polyethylenenaphthalenedicarboxylate (PEN 14991 available from Eastman Chemicals,Kingsport, Tenn.).

The extruded compositions were analyzed by Differential ScanningCalorimetry (DSC). The individual polyesters present in the extrusioncoating compositions also were tested by DSC. In particular, a TAInstrument Model 2920 Differential Scanning Calorimetry instrument wasused to determine if the extruded products were blends or alloys of thestarting polyester. Samples were prepared for analysis by cutting asmall sample (e.g., 10 to 15 mg in weight) from an extruded film with arazor blade. The sample was placed into an aluminum DSC sample pan,covered, and sealed. The sample pan then was placed into the DSCapparatus and cooled to −50° C. Each sample was heated at a rate of 20°C. per minute to 300° C.

After a first heating, each sample was rapidly cooled from its meltpoint down to −50° C. A second heating was performed on each sample, andthe thermal transitions were recorded. Each extruded composition wasanalyzed under the same conditions as the individual polyesters. Thefirst heating of each extruded composition revealed multiple transitionsthat were matched with transitions attributed to the startingpolyesters. For example, a P1500/-PEN sample conditioned at 300° C. for2 minutes provided a Tg that matched the DYNAPOL starting component anda Tg that matched the PEN starting component. No other thermaltransitions were detected. The thermal analysis results showed that theextruded composition was a blend of the starting polyesters. A polymeralloy was not formed from the individual polyesters. Thermal analysesusing different conditioning times all revealed similar findings.Thermal analysis of the DYNAPOL 1500/-CARIPAK P76 extrusion coatingcompositions after extrusion also was a blend of the startingpolyesters, as opposed to an alloy.

Extrusion coating compositions of Examples 1-31 and Comparative ExamplesA and B were extruded onto a metal substrate to provide a coated metalsubstrate having an adherent barrier layer of an extruded composition.Typically, the compositions are applied to a sheet or coil of a metalsubstrate moving relative to an extruder which applies the compositionto the metal substrate. The extruder comprises a screw to transfer themolten composition, and a die to apply the composition to the metalsubstrate at a predetermined thickness. The extruder applies theextrusion coating composition to the metal substrate as a layer of about1 to about 40, and preferably about 2 to about 30, microns. To achievethe full advantage of the present invention, the extruded coatingcomposition is about 1 to about 10 microns thick.

The coated metal substrates then were tested for use as the interiorsurface of a food or beverage container. As will be demonstrated morefully hereafter, an extruded coating composition resulting fromextruding an extrusion coating composition of the present invention issuitable as the interior coating of a metal container for food orbeverages. A present extrusion coating composition provided excellentextruded coatings in the absence of a curing step.

In particular, an extrusion coating composition of the present inventioncan be applied to essentially any metal substrate. Nonlimiting examplesof metal substrates are aluminum, tin-free steel, tinplate, steel,zinc-plated steel, zinc alloy-plated steel, lead-plated steel, leadalloy-plated steel, aluminum-plated steel, aluminum alloy-plated steel,and stainless steel.

In the extrusion coating method, the extrusion coating composition isslowly and carefully melted by first heating the composition at about100° C. to about 120° C., then slowly increasing the temperature toabout 180° C. to about 240° C. to completely melt the extrusion coatingcomposition. The upper temperature is not especially limited, but issufficiently high to melt the composition. The composition should not beheated to a temperature far above the melting point (i.e., greater thanabout 100 C.° above the melting point) in order to minimize or avoid,reactions between the first and second polyesters, between thepolyesters and the optional modifying resin, or degradation of thepolyesters.

An important feature of the present extrusion coating compositions isstability of the composition at the melting point temperature. Extrusioncoating compositions of the present invention increase in viscosity byless than 100 Pa.s when maintained at, or above, the melting point fortwenty minutes. This slight increase in viscosity shows that thepolyesters and epoxy are not reacting, i.e., undergoing cure, at themelting point to form a crosslinked resin, and that the polyesters arenot degrading. If crosslinking did occur, the viscosity would risedramatically, and it would be difficult to impossible to extrude theextrusion coating composition onto a metal substrate. A decrease inviscosity indicates that the polyesters were degrading at the meltingpoint.

In addition, the metal is heated prior to extrusion to a temperature ofabout 60° C. to about 300° C., preferably about 80° C. to about 260° C.Preheating of the metal substrate is important to achieve sufficientflow of the extrusion coating composition on the metal substrate, and toachieve adhesion of the extruded composition to the metal substrate.

The extrusion coating composition does not cure or crosslink to anysubstantial extent during, or after, extrusion onto the heatedsubstrate. Therefore, a step of curing the extruded composition at anelevated temperature is omitted. However, to optimize the properties ofthe extruded composition, the coated metal substrate, either before orafter cooling, preferentially is subjected to a postextrusion heatingstep conducted at about 120° C. to about 550° C. (peak metaltemperature) for about 5 to about 30 seconds, and preferably about 180°C. to about 450° C. for about 10 to about 20 seconds.

The resulting extruded coating compositions had a smooth, glossyappearance, and were defect free. The extruded coatings had goodadhesion and exhibited good barrier and anticorrosion properties.

In particular, the present extrusion coating compositions provide ahighly protective, self-lubricating layer when extruded onto a metalsubstrate. The compositions can be extruded onto metal coils or sheetsat a high speed to provide a layer of extruded composition having athickness of about 1 to about 40 microns, preferably about 2 to about 30microns. For many applications, the extruded composition has a thicknessof about 1 to about 10 microns. Typically, the extruder has alength-to-diameter ratio (L/D) of about 10:1 to about 40:1, andpreferably about 15:1 to abut 30:1. The extruder can contain a singlescrew, or twin screws, either conrotatory or counter rotatory. Theextruded composition outperforms liquid and powder coating compositions,and reduces the cost of applying a thin protective coating on a metalsubstrate.

Overall, an extrusion coating composition of the present inventiondemonstrates the advantages of optionally eliminating chemicalpretreatment of the metal substrate; optionally substituting a smallinduction oven for preheating the metal substrate and for postheating inplace of a large convection oven for drying a liquid composition; usinga solid composition containing no organic compounds in place of a liquidcontaining organic solvents; elimination of lubricating stations; andelimination of solvent incinerators.

Various tests were performed on the extrusion coating compositions.These tests showed that the present extrusion coating compositionsexhibit coating properties at least equal to current commercialcompositions used for similar practical applications. The datasummarized below illustrates that an extrusion coating composition ofthe present invention provides an extruded coating composition useful onthe interior or exterior coating of a food or beverage container.

In particular, a coating composition for a metal container mustdemonstrate excellent adhesion and flexibility because metal containersare manufactured by first coating flat sheets of the metal substrate,then forming the coated sheets into a desired shape. Coatings havingpoor adhesion properties can separate from the metal substrate duringthe shaping process. A lack of adhesion, therefore, can adversely affectthe ability of the cured coating composition to inhibit corrosion of themetal substrate. A present extrusion coating composition exhibits anexcellent adhesion to a metal substrate, and, therefore, a coating canbe extruded onto a metal substrate, and the metal substrate subsequentlycan be deformed without adversely affecting continuity of the coatingfilm.

The extruded coating compositions also have excellent flexibility.Flexibility is an important property of a polymeric coating because themetal substrate is coated prior to stamping or otherwise shaping themetal substrate into a desired metal article, such as a metal container.The coated metal substrate undergoes severe deformations during theshaping process, and if a coating lacks sufficient flexibility, thecoating can form cracks or fractures. Such cracks result in corrosion ofthe metal substrate because the aqueous contents of the container havegreater access to the metal substrate. Metal substrates coated with apresent extrusion coating composition were deformed into the shape of ametal can. No cracks or fractures were observed. In addition, aspreviously described, an extruded coating provided by an extrusioncoating composition of the present invention is sufficiently adherent tothe metal substrate, and remains sufficiently adherent during processinginto a metal article, and, therefore, further enhances corrosioninhibition.

The tests summarized below demonstrate that a present extruded coatingcomposition maintains adhesion to the metal substrate, is flexible, issufficiently hard, and, therefore, is scratch and mar resistant, resistsblush, and resists chemical attack. Such a combination of advantages arenecessary, or at least desirable, in a coating applied to the interiorof food and beverage containers.

The above-described advantages made an extrusion coating composition ofthe present invention useful for application on the interior orextrusion surface of a variety of metal articles, such as for theinterior of metal containers for foods and beverages. A presentextrusion coating composition is especially useful as a coating on ametal container that holds taste-sensitive foods or beverages, likebeer, because the extruded coating composition is essentially free ofcomponents that affect the taste of the food or beverage.

The tests performed on metal substrates coated with an extrusion coatingcomposition of the present invention are well known to persons skilledin the art and are summarized as follows:

DOWFAX Test (Crosshatch Test)

Coated samples were immersed in a boiling 1.67% aqueous solution ofDOWFAX 2A1 surfactant for 15 minutes, rinsed in hot water, and dried.The samples then were crosshatched, taped, and rated for adhesionaccording to the following system:

0—perfect

1—very slight pickoff from edges of squares

2—slight pickoff (1-2%)

3—moderate pickoff (2-50%)

4—severe pickoff (>50%)

5—very severe, crosshatching removes the coating.

The samples also were rated for blushing as follows:

0—perfect

1—very slight haze on surface

2—slight cloudy appearance

3—moderate cloudy appearance

4—very cloudy and dull appearance, possible discoloration.

The blush resistance test demonstrates the ability of an extrudedcoating to resist attack by a hot detergent solution. Adhesion is testedby the crosshatch adhesion test wherein razor blades make perpendicularcrosshatch patterns in an applied coating. Adhesive tape is applied tothe crosshatch patterns, then the adhesive tape is removed at a 90°angle in a fast movement. The amount of extruded coating remaining asthe metal substrate then is determined.

MSE Test

Coated samples are immersed in equal parts of 4% (v/v) acetic acid, 6%(v/v) lactic acid, and 6% (wt/v) NaCl for 15 minutes at 100° C. Thesamples are rated as in the above-described DOWFAX test.

Hot Water Test

Coating samples were immersed in water for 30 minutes at 65° C. Thesamples were rated as in the DOWFAX test.

Processing Test Solutions Solution D Demineralized water Solution S¹ 40g concentrated acetic acid 24 g gelatin 24 g sodium chloride 0.4 gcrystalline sodium sulfide (Na₂S · 9 H₂O) q.s. water to about 800 mlSolution R¹ 16 g citric acid crystals 3.2 g vitamin C (ascorbic acid)q.s. water to about 800 ml Solution O¹ 16 g citric acid crystals 0.2 gH₂O₂ hydrogen peroxide (30% sol.) 0.8 g NH₄NO₃ ammonium nitrate q.s.water to about 800 ml ¹The test solutions D, S, R, O have been chosenfor cans containing a wide variety of food filling; these tests areconducted for 1 hour at 121° C.

Water Pasteurization

After immersion in 180° F. water for 30 minutes, the coated panels weretested for 25 lb. reverse impact, blush, and pencil hardness.

Retort Test

The retort test was performed to evaluate the resistance and adhesion ofcoatings under food processing conditions (90 minutes @ 250° F. and 15psi).

4-Corner Cup

This is a shallow, drawn can body of approximately rectangular shape.Each of the four corners is curved, and each curved corner has adifferent diameter. The 4-Corner Cup is prepared from a metal substratehaving a coating applied thereon before the can body is formed.

Mi (Lactic Acid), Cy (Cysteine) and NaCl/HAC (Sodium Chloride/AceticAcid) Tests

A coated substrate is formed into a 4-Corner Cup container, then a testsolution is added to the 4-Corner Cup and held at 120° C. for one hour.The lactic acid solution is a 1% aqueous lactic acid solution. Thecysteine solution contains 0.45 g cysteine and about 10 g of phosphateper liter of aqueous solution. The NaCl/HAC solution contains 2% sodiumchloride and 3% acetic acid in water.

Civo-Test

A 4-Corner Cup container is placed in a larger container, and the largercontainer is filled with a 3% aqueous acetic acid solution. The largecontainer is heated to 70° C. and held for two hours. The container thenis cooled, and stored for 10 days at 40° C. The 4-Corner Cup containerthen is inspected for defects.

Examples 1 through 20 illustrate extrusion coating compositionscontaining blends of a first and second polyester having Tg's thatdiffer by at least 5 C.°, either containing or free of an optionalmodifying polymer. Examples 21-29 illustrate extrusion coatingcompositions containing blends of PET and PBT polyesters, and blends ofPET and PBT polyesters with a copolyester, each containing and free of amodifying resin. Examples 30 and 31 illustrate pigmented extrusioncoating compositions.

The properties of extruded coating compositions resulting from theextrusion coating compositions of Comparative Examples A and B and ofExamples 1-31 are set forth in the following Table 1. In general, theresults summarized in Table 1 show PET and PBT polyesters improve theadhesion of a copolyester to a metal substrate (Examples 1-20).Accordingly, a modifying resin, which promotes adhesion, can be excludedfrom the extrusion coating composition. Examples 21-30 show that blendsof polyesters gave good film properties, and that including a low amountof a copolyester improved performance, e.g., less blushing was observed.The ability to use a polyester, like PET or PBT, has the benefits oflowering the cost of the composition without adversely affectingextruded coating composition performance, and providing the ability todesign extrusion coating compositions having a viscosity suitable forspecific application apparatus and methods.

TABLE 1 Comp. Comp. Ex. A Ex. B Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex.7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17Ex. 18 Ex. 19 Ex. 20 Film Thickness 12-16 9-12 12 10-15 8-12 8-13 9-138-14 11-16 5-11 10-13 9-12 10-15 10-17 8-15 5-11 7-13 9-12 8-10 10∫1710-12 11-16 (microns) Cross Hedge 0 3 1 1 0 0 0 2 0 0 0 0 0 0 0 0 0 0 00 0 0 (GT) Dur-O-Test 1400 1200 2000 1800 1500 1500 2000 1500 2000 18001800 1800 1800 1800 2000 2000 2000 1800 1900 1800 1800 1800 (grams)Wedge Bend 100 100 100 m/c¹ m/c m/c 100 100 100 m/c m/c m/c 100 100 100100 100 m/c m/c 100 100 100 (%) Four Corner O.K. O.K. O.K. m/c  O.K. m.cO.K. O.K. O.K. m/c m/c m/c O.K. O.K. O.K. O.K. O.K. m/c m/c O.K. O.K.O.K. Cup Hot Water 30 min. at 65° C. GT 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 Blushing 0 0 0 0 0 0 0 0 2 0 1 3 1 1 0 0 0 3 0 1 1 0 MSETest 15 min. at 100° C. GT 0 0 1 0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0Blushing 0 0 0 0 m/c 0 0 0 0 2 1 0 1 0 0 2 Boiling Water 30 min. WedgeBend 100 100 100 0 m/c 0 m/c 0 m/c 100 90 100 0 m/c 0 m/c 0 m/c 100 100100 100 100 0 m/c 0 m/c 80 100 100 Four Corner 2 1 0 0 m/c 0 m/c 0 m/c 00 2 1 m/c 4 4 2 1 3 1 0 4 4 1 1 1 Cup Sterilization Water 1 hr. at 121°C. Wedge Bend 100 100 100 0 m/c 0 m/c 0 m/c 100 100 100 0 m/c 0 m/c 0m/c 100 100 100 100 0 m/c 0 m/c 0 m/c 90 100 100 Four Corner 1 1 0 0 m/c0 m/c 0 m/c 0 0 2 1 m/c 4 4 1 1 3 1 0 3 4 1 1 1 Cup Sterilization 1 hr.at 121° C. GT/Blushing D 0/0 0/0 0/0 0/0 1/0 0/0 0/0 0/0 0/2 0/1 1/2 0/20/1 0/1 0/2 0/0 0/0 3/3 1/1 0/1 0/1 0/1 GT/Blushing m/c S 0/0 0/0 0/00/0 0/0 0/0 0/0 0/0 0/2 0/0 1/2 0/3 0/1 0/2 0/2 0/0 0/0 3/3 0/2 0/1 0/10/1 GT/Blushing m/c R 0/0 5/0 1/0 0/0 5/0 5/0 5/0 5/0 4/2 0/0 2/2 4/25/1 5/1 0/3 2/0 3/0 4/0 5/2 5/2 5/1 5/1 GT/Blushing m/c O 0/0 5/0 1/01/0 5/0 5/0 5/0 5/0 2/2 0/0 2/2 5/2 5/1 5/1 0/3 1/0 4/0 3/3 5/2 5/2 5/15/1 GT/Blushing m/c Sterilization 1 hr. at 128° C. GT/Blushing LacticAcid 0/1 1/1 0/1 0/0 1/1 1/2 0/2 3/0 0/3 0/1 2/2 1/2 0/1 0/2 0/4 0/2 0/13/3 1/3 0/3 0/0 0/1 m/c Acetic 0/0 0/0 0/1 0/1 1/2 0/1 0/1 0/0 0/3 0/22/1 1/2 0/1 0/2 0/3 0/1 0/1 2/3 1/1 0/2 0/1 0/1 acid/NaCl m/c ¹⁾m/c =micro cracks

TABLE 1 Ex. 21 Ex. 22 Ex. 23 Ex. 24 Ex. 25 Ex. 26 Ex. 27 Ex. 28 Ex. 29Ex. 30 Ex. 31 Film Thickness 6-10 8-13 8-12 5-10 10-13 9-13 8-11 7-1110-12 8-15 5-11 (microns) Cross Hedge 0 0 0 0 0 0 0 0 0 0 0 (GT)Dur-O-Test 2000 1800 1800 1800 1800 1800 1600 1700 1800 2000 2000(grams) Wedge Bend m/c m/c m/c m/c m/c m/c m/c m/c m/c 100 100 (%) FourCorner Cup O.K. m/c m/c m/c m/c m/c O.K. m/c m/c O.K. O.K. Hot Water 30min. at 100° C. GT 0 0 0 0 0 0 0 0 0 0 0 Blushing 0 0 1 0 2 3 0 0 1 0 0MSE Test 15 min. at 100° C. GT 0 0 0 0 1 0 0 0 0 0 0 Blushing 0 0 1 0 12 0 0 1 2 0 Boiling Water 30 min. Wedge Bend 0 m/c 0 m/c 0 m/c 0 m/c 700 m/c 0 m/c 0 m/c 0 m/c 100 100 Four Corner Cup 0 m/c 2 3 0 m/c 3 4 0m/c 2 4 3 1 Sterilization Water 1 hr. at 121° C. Wedge Bend 0 m/c 0 m/c0 m/c 0 m/c 90 100 0 m/c 0 m/c 0 m/c 100 100 Four Corner Cup 0 m/c 1 2 1m/c 3 4 0 m/c 1 3 3 1 Sterilization 1 hr. at 121° C. GT/Blushing D 0/00/0 1/2 1/0 1/2 1/3 1/0 0/0 1/2 0/2 0/0 GT/Blushing S 1/0 1/1 0/2 0/01/1 1/3 0/0 1/0 1/2 0/2 0/0 GT/Blushing R 1/0 2/0 2/2 5/0 5/2 2/4 5/05/0 2/2 0/3 2/0 GT/Blushing O 2/0 2/0 2/2 5/0 5/2 2/4 5/0 5/0 2/1 0/31/0 GT/Blushing Sterilization 1 hr. at 128° C. GT/Blushing Lactic Acid1/0 1/0 1/3 1/0 5/2 0/4 1/2 1/1 1/3 0/4 0/2 Acetic acid/NaCl 0/0 0/0 1/21/0 5/2 0/4 0/1 1/1 1/3 0/3 0/1

Obviously, many modifications and variations of the invention ashereinbefore set forth can be made without departing from the spirit andscope thereof, and, therefore, only such limitations should be imposedas are indicated by the appended claims.

What is claimed is:
 1. A method of coating a metal substrate comprising:(a) heating the metal substrate to a temperature of about 60° C. toabout 300° C. to provide a preheated metal substrate; (b) heating asolid, thermoplastic coating composition to a sufficient temperature tomelt the coating composition and provide a molten coating composition,said coating composition comprising: (i) about 50% to about 100%, bytotal weight of the composition, of a blend of polyesters comprising (A)a first polyester having a weight average molecular weight of about10,000 to about 80,000 and a glass transition temperature of greaterthan 45° C. to about 100° C., and (B) a second polyester having a weightaverage molecular weight of about 10,000 to about 70,000 and a glasstransition temperature of about −10° C. to about 45° C., wherein the Tgof the first polyester is about 5 C.° to about 60 C.° higher than the Tgof the second polyester, and wherein the first polyester and the secondpolyester are present in a weight range of about 1 to 6 to about 1 to 9;(ii) 0% to about 25%, by total weight of the composition, of a modifyingresin selected from the group consisting of an epoxy or phenoxy resinhaving an epoxy equivalent weight of about 500 to about 15,000, anacrylic resin having a weight average molecular weight of about 15,000to about 100,000, a polyolefin resin having a weight average molecularweight of about 15,000 to about 1,000,000, and mixtures thereof; (c)extruding the molten coating composition onto a surface of the preheatedmetal substrate to provide a layer of molten coating composition aboutone to about 40 microns thick on the preheated metal substrate andprovide a coated metal substrate; and (d) heating the coated metalsubstrate at a temperature of about 120° C. to about 550° C. for about 5to about 30 seconds.
 2. The method of claim 1 further comprising thestep of allowing the coated metal substrate to cool between steps (c)and (d).
 3. A metal article formed from the coated metal substrate ofclaim
 2. 4. The method of claim 1 wherein the thermoplastic coatingcomposition further comprises: (iii) 0% to about 50%, by total weight ofthe composition, of an inorganic filler; and (iv) 0% to about 4%, bytotal weight of the composition, of a flow control agent.
 5. The methodof claim 4 wherein the coating composition comprises 0% to about 30% bytotal weight of the composition of an inorganic filler.
 6. The method ofclaim 4 wherein the inorganic filler is selected from the groupconsisting of clay, mica, aluminum silicate, fumed silica, magnesiumoxide, zinc oxide, barium oxide, calcium sulfate, calcium oxide,aluminum oxide, magnesium aluminum oxide, zinc aluminum oxide, magnesiumtitanium oxide, iron titanium oxide, calcium titanium oxide, andmixtures thereof.
 7. The method of claim 4 wherein the flow controlagent comprises an acrylic resin.
 8. The method of claim 1 wherein thethermoplastic coating composition further comprises up to about 50%, bytotal weight of the composition, of a second modifying polymer.
 9. Themethod of claim 8 wherein the second modifying polymer is athermoplastic polymer.
 10. The method of claim 8 wherein the secondmodifying polymer is a thermoset polymer.
 11. The method of claim 8wherein the second modifying polymer is selected from the groupconsisting of a carboxylated polyester, a carboxylated polyolefin, apolyamide, a fluorocarbon resin, a polycarbonate, a styrene resin, anacrylonitrile-butadiene-styrene resin, a chlorinated polyether, aurethane resin, and mixtures thereof.
 12. The method of claim 8 whereinthe second modifying polymer is capable of providing a coated metalsubstrate having a nongloss appearance.
 13. The method of claim 1wherein the metal substrate is selected from the group consisting ofaluminum, tin-free steel, tinplate, steel, zinc-plated steel, zincalloy-plated steel, lead-plated steel, lead alloy-plated steel,aluminum-plated steel, aluminum alloy-plated steel, and stainless steel.14. The method of claim 1 wherein the coating composition is heated instep (b) to a temperature of about 180° C. to about 350° C.
 15. Themethod of claim 14 wherein the coating composition is heated to amaximum of 100° C. above a melting point of the coating composition. 16.The method of claim 1 wherein the coating composition is free of organicsolvents.
 17. The method of claim 1 wherein the Tg of the firstpolyester is about 15 C.° to about 35 C.° higher than the Tg of thesecond polyester.
 18. The method of claim 1 wherein the coatingcomposition comprises about 60% to about 85%, by total weight of thecomposition, of a polyester.
 19. The method of clam 1 wherein the firstand second polyesters have a melt viscosity of about 200 to about 3000Pa.s and a softening point of about 120° C. to about 200° C.
 20. Themethod of claim 1 wherein the first and second polyesters have an acidnumber of 0 to about 150 mg KOH/g and a hydroxyl number of 0 to about150 mg KOH/g.
 21. The method of claim 1 wherein the first and secondpolyesters have a melt viscosity of about 250 to about 2000 Pa.s. 22.The method of claim 1 wherein the first and second polyesters have amelt flow index of about 800 g/10 min at 200° C. or about 5 g/10 min. at280° C.
 23. The method of claim 1 wherein the first and secondpolyesters comprise condensation product of (i) a dicarboxylic acid oran esterifiable derivative of a dicarboxylic acid, and (ii) an aliphaticdiol, wherein at least 60 mol % of the dicarboxylic acid or dicarboxylicacid derivative is an aromatic dicarboxylic acid.
 24. The method ofclaim 23 wherein the aromatic dicarboxylic acid is selected from thegroup consisting of phthalic acid, isophthalic acid, terephthalic acid,a naphthalene dicarboxylic acid, and mixtures thereof.
 25. The method ofclaim 1 wherein the first and second polyesters comprise the reactionproduct of (i) a dicarboxylic acid or an esterifiable derivative of adicarboxylic acid, and (ii) a low molecular weight epoxy resin having anEEW of about 150 to about
 500. 26. The method of claim 1 wherein thefirst and second polyesters are selected from the group consisting of apolyethylene terephthalate, a polybutylene terephthalate, a polyethylenenaphthanate, a polybutylene naphthanate, a copolyester, apolytrimethylene terephthalate, a polytrimethylene naphthanate, andmixtures thereof.
 27. The method of claim 1 wherein the coatingcomposition comprises about 2% to about 20%, by total weight of thecomposition, of a modifying resin.
 28. The method of claim 1 wherein themodifying resin comprises an epoxy resin having an epoxy equivalentweight of about 2000 to about
 8000. 29. The method of claim 1 whereinthe epoxy resin is a solid material containing an average of about 1.5to about 2.5 epoxy groups per molecule of the epoxy resin.
 30. Themethod of claim 1 wherein the epoxy resin is a solid material containingan average of about 2.5 to about 6 epoxy groups per molecule of theepoxy resin.
 31. The method of claim 1 wherein the epoxy resin comprisesa mixture of an epoxy resin having about 1.5 to about 2.5 epoxy groupsper molecule of the epoxy resin and an epoxy resin having about 2.5 toabout 6 epoxy groups per molecule of the epoxy resin.
 32. The method ofclaim 1 wherein the epoxy resin is an aromatic epoxy resin.
 33. Themethod of claim 32 wherein the aromatic epoxy resin is based onbisphenol A or bisphenol F.
 34. The method of claim 1 wherein themodifying resin is an acrylic resin having a weight average molecularweight of about 20,000 to about 80,000.
 35. The method of claim 1wherein the acrylic resin is a homopolymer or a copolymer of homopolymerand copolymers of acrylic acid, methacrylic acid, esters of acrylicacid, esters of methacrylic acid, acrylamides, and methacrylamides. 36.The method of claim 1 wherein the modifying resin is a polyolefin resinhaving a weight average molecular weight of about 25,000 to about750,000.
 37. The method of claim 1 wherein the polyolefin resin is ahomopolymer or a copolymer of ethylene, propylene, ethylene, propyleneblends, 1-butene, and 1-pentene.
 38. The method of claim 1 wherein thepolyolefin comprises a functionalized olefin.
 39. The method of claim 1wherein the molten coating composition is extruded onto opposingsurfaces of the preheated metal substrate.
 40. The method of claim 1wherein the layer of molten coating composition is about 2 to about 30microns thick.
 41. The method of claim 1 wherein the layer of moltencoating composition is about 1 to about 10 microns thick.
 42. The methodof claim 1 wherein the molten coating composition is extruded onto thepreheated metal substrate by an extruder comprising a screw and die,wherein the preheated metal substrate is moving relative to the die. 43.The method of claim 42 wherein the screw is a single screw, aconrotatory twin screw, or a counter rotatory twin screw.
 44. The methodof claim 1 wherein the heating step is performed at 180° C. to about450° C. for about 15 to about 20 seconds.
 45. The method of claim 1wherein the thermoplastic coating composition further comprises apigment, an organic dye, or a mixture thereof.
 46. A metal articleformed from the coated metal substrate of claim
 1. 47. The article ofclaim 1 wherein the article is a can body, a can end, or a metal closurefor a container.